The base A0 size of paper is defined as having an area of 1 m2. Rounded to the nearest millimetre,
the A0 paper size is 841 by 1,189 millimetres (33.1 in × 46.8 in). Successive paper sizes in the series A1, A2, A3,
and so forth, are defined by halving the preceding paper size across the larger dimension.

It measures 8.5 by 11 inches (215.9 mm x 279.4 mm). US Letter size is a recognized standard adopted by the American
National Standards Institute (ANSI) whereas the A4 is the International Standard (ISO) used in most countries.